Write down the plugin nobody documented, and stop pointing at a deleted branch

PLUGINS.md listed 14 of the 15 bundled plugins. `backups` had no row: the
per-asset configuration backups the collector takes off shopfloor machines,
kind-pluggable, with view and download as separate permissions because a .reg
is a restorable artifact. Added, with the pointer that separates it from
BACKUP-RESTORE.md, which is the database and a different subject.

PLUGIN-LAB-PRINTEDPARTS said the finished implementation lives on branch
feat/printedparts-plugin. That branch was deleted in the v0.8.0 cleanup; only
the 17 lab-stage tags survived, and they are enough. Says so now, and warns
that the tags are a record of contract 0.11-0.13 rather than today's
conventions. Its manifest example carried core_version >=0.12.0 against a
shipped >=0.16.0.

The printer-assignment proposal still said the client script is not built. It
shipped and released, across v0.11.3 and v0.12.0, and the collector half that
contradicted the assigned-vs-observed split was retired in adc7ef4.

PROJECT-MAP regenerated for the test count.

docs/PROJECT-MAP.md

@@ -86,6 +86,6 @@ Manifest-less directories under `plugins/` are core frontend surface and always
 
 ## Size
 
-- test functions defined: **1233** (parametrised cases collect higher)
+- test functions defined: **1236** (parametrised cases collect higher)
 - documented API paths: **285** (`docs/openapi.json`, regenerate with `scripts/gen_openapi.py`)